This invention relates to a process for the preparation of nitric acid and, more particularly to an atmospheric pressure process for the conversion of nitrogen oxides to nitric acid in a relatively small reaction zone as compared to those employed in existing atmospheric pressure processes.
Most commercially available nitric acid is produced by the first step of oxidizing ammonia to form nitrogen oxides, followed by absorption of the nitrogen oxides into water to form nitric acid. In the first step, the ammonia is initially converted to nitric oxide by oxidizing the ammonia in the presence of excess oxygen over a suitable catalyst such as a platinum gauze catalyst. The ammonia oxidation is exothermic, with water being a by-product. In balanced form, the equation for this reaction is: EQU NH.sub.3 + 1.25O.sub.2 = NO + 1.5H.sub.2 O (1)
the nitric oxide formed in reaction (1) is then oxidized to form nitrogen dioxide. The reaction is relatively slow and homogeneous and proceeds according to the equation: EQU 2NO.sub.(g) + O.sub.2(g) = 2NO.sub.2 ( 2)
below 150.degree. C, the equilibrium constant strongly favors the formation of nitrogen dioxide (and its dimer, nitrogen tetroxide) so that almost all nitric oxide will combine with any oxygen present to form nitrogen dioxide (and its dimer nitrogen tetroxide).
In the latter step, nitrogen dioxide (or its dimer nitrogen tetroxide) is absorbed in water to produce the nitric acid. The equation for this reaction is: EQU 3NO.sub.2(g) + H.sub.2 O.sub.(liq.) = 2 HNO.sub.3(aq) + NO.sub.(g) ( 3)
As can be seen, additional nitric oxide is produced in this reaction. The nitric oxide produced in this reaction then combines with any oxygen present to form nitrogen dioxide (and its dimer nitrogen tetroxide) according to reaction (2). The nitrogen dioxide thus formed absorbs in any water present and additional nitric oxide is released. For every three moles of nitrogen dioxide that is converted to nitric acid in reaction (3), one mole of nitric oxide is released. As the concentration of nitric oxide gets smaller and smaller, reaction (2) goes more and more slowly; and, in fact, never of itself goes to completion. However, in any commercial process the last traces of nitric oxide should be removed from the exhaust gases so that these gases will be within the standards set by the Environmental Protection Agency for pollutants and also to minimize the economic penalty paid when the nitric oxide, a reactant, is lost to the atmosphere. Heretofore, removal of the last nitric oxide has been accomplished by the use of increased operation pressures and/or the use of large reactor volumes.
The reaction between nitric oxide and oxygen is a third order reaction and its reaction rate will increase as the square of the pressure. The residence time for a given quantity of gas (by weight) in passing through a reactor of given volume increases in direct proportion to the pressure. It follows that the volume of the oxidation space to accomplish a given degree of oxidation of nitric oxide would be inversely proportional to the cube of the pressure. For a pressure of 8 atm, the reactor volume required would be only 1/512 of that necessary at atmospheric pressure. Of course, the pressure equipment required is expensive to construct and maintain.
One process utilizing elevated operating pressures is that developed by DuPont. A good summary of this process and its development is given in T. H. Chilton, Chem. Eng. Prog. Monograph Series No. 3, Vol. 56, Am. Inst. Chem. Eng., N.Y. (1960).
In a plant employing the "DuPont Process", air is compressed to about between 50 and 125 psig, preheated to about 250.degree. C, and mixed with ammonia vapor. The mixture, containing about 10% ammonia by volume, flows down through a pack of flat gauzes producing nitric oxide at an efficiency of about 95% at a temperature of about 900.degree. C. The hot gas leaving the gauze is cooled by exchange with the feed air and in a tail-gas reheater before flowing to a cooler-condenser. Weak acid produced in the condenser is pumped to an intermediate tray of the absorption tower while the uncondensed process gas flows in the bottom. The absorption tower consists of a series of bubble cap trays provided with cooling coils for removing the heat of reaction. As the gas flows up the tower countercurrent to the acid flow, nitrogen dioxide dissolves in water forming nitric acid and releasing nitric oxide, which is reoxidized in the space between the trays by the excess oxygen present. Steam condensate is added to the top of the tower as the absorbent; dissolved nitrogen oxides are removed from the product acid by contact with secondary air in a "bleaching" tower. The tail gas leaving the absorption tower is reheated to about 250.degree. C by exchange with the process gas and then expanded through a gas engine which provides up to about 40% of the power required for driving the reciprocating air compressors. Typically such a plant will produce 250 tons a day of 100% nitric acid at a volumetric efficiency of about 85 to 90 pounds of 100% nitric acid per day per cubic foot of reactor.
The alternative to the use of elevated pressure to minimize the amount of nitric oxide in the exhaust gases is to use a series of reactors to oxidize the nitric oxide to nitrogen dioxide. In this process, the exhaust gas containing the regenerated nitric oxide is fed into a second reactor where it is contacted with additional oxygen and water to form nitric acid and, of course, additional regenerated nitric oxide, which is in turn fed into a third reactor; and the process is repeated until the nitric oxide level in the exhaust is practically eliminated.
As can be appreciated, each of these techniques has its drawbacks. The use of large reactor machines requires not only considerable capital investment, but also increases the retention time of gas in the reactor. Increasing the operating pressure also increases the gas retention time and further necessitates the use of pressure equipment which is costly to install and maintain.
It is, therefore, an object of this invention to provide a process for the preparation of nitric acide from nitrogen oxides which virtually eliminates regenerated nitrogen oxides in the process exhaust gases.
Another object of this invention provides a process of the type described herein which can be operated at essentially atmospheric pressure with relatively small reactor volumes.
It is yet another object of this invention to provide such a process that operates in a single gas-liquid contacting reaction zone.
Still another object of this invention lies in the provision of a process as described herein which is capable of achieving yields of 85% and greater, of 100% nitric acid.
A further object of this invention is to provide such a process characterized by its ability to effectively react dilue concentrations of nitrogen oxides.
Yet another object provides an integral process as described herein for efficiently and economically converting ammonia to nitric acid.